In order to improve driving safety of a vehicle, there have been recently proposed various devices that can collect information such as air pressure, temperature, etc. in a tire during running to monitor a condition of the tire. In general, these devices include a tire-mounted unit and a vehicle-mounted unit, the tire-mounted unit having a pressure sensor for detecting pressure within the tire and/or a temperature sensor for detecting temperature in the tire, and a transmitter for transmitting a signal detected by the sensor to the vehicle side via an antenna, the vehicle-mounted unit having a receiver for receiving the detected signal transmitted from the transmitter via an antenna, a processor for processing the detected signal received by the receiver and the like; communication is performed by electromagnetic waves between the tire-mounted unit placed in the tire and the vehicle-mounted unit (see patent document 1, for example).
In order to use the devices with no license and no notification, very weak radio waves that do not conflict with the restrictions of the radiowave regulation are employed for the electromagnetic waves used for communication between the tire-mounted unit and the vehicle-mounted unit. The antenna of the vehicle-mounted unit is placed in a tire house near the tire in order to effectively receive a radio wave.
The antenna of the vehicle-mounted unit has an antenna element, and the antenna element is normally contained in an antenna case for protection. However, when a stone, foreign object or the like flipped up by the tire in the tire house strikes on the antenna, it may bring the antenna case to destruction and further break the antenna element to thereby impair the communication function. If the antenna element is broken to thereby lose communication ability, the replacement of the antenna is required; the replacement operation is time consuming, incurring a decrease in vehicle operation efficiency.
Patent Document 1: Japanese Patent Application Kokai Publication No. 2003-63220